An air impact driver provided for screwing a building material of a plaster board or the like is mounted with a mechanical type safety apparatus by a contact arm similar to a nail striker. The contact arm is an arm formed in a crank-like shape along outer shapes of a nose and a cylinder housing of the air impact driver which is slidable in parallel with the nose, a front end thereof is projected frontward from the nose and other end reaches a front face of a trigger lever of a main body of the air impact driver.
A trigger lever is attached with a pivotable free arm and when the contact arm is pressed to a side of the main body of the air impact driver, a front end portion of the free arm attached to the trigger lever is pressed by the contact arm to be proximate to a stem of a trigger valve. When the trigger lever is pulled under the state, the free arm is pivoted in a direction of the trigger valve by constituting a fulcrum by the front end portion along with the trigger lever and the stem of the trigger valve is pressed by the free arm to start the air impact driver. Further, even when first, the trigger lever is pulled and thereafter the contact arm is pressed to an object face of screwing, the free arm presses the stem of the trigger arm to start to the air impact driver similar to the above-described operation.
In this way, the trigger lever and the contact arm are constituted to cooperatively make the trigger valve ON, when only the trigger lever is operated, the free arm does not reach a position of the stem of the trigger arm and the air impact driver cannot be started to thereby prevent the air impact driver from being started by erroneously operating the trigger lever.
According to the mechanical type safety mechanism of the above conventional art, the contact arm passes a side face of the cylinder-housing and therefore, a width of a total of the air impact driver is widened and there is a case in which it is difficult to strike a screw to a corner portion or a location having a narrow width. Further, according to the constitution in which the front end of the free arm attached to the trigger lever is pressed up by sliding the contact arm and the total of the free arm is moved by pulling the trigger lever to thereby press the stem of the trigger valve, there poses a problem that the stroke of pressing the contact arm is prolonged and therefore, operability is not excellent, further, operation of one cycle is devoid of swiftness.
Further, as other problem, there is a case in which the contact arm cannot be slid by clogging a plaster powder produced in screwing between a slide guide supporting the long contact arm and the contact arm and when the contact arm is not returned from a pressing position to an initial position, there poses a problem that when the trigger lever is erroneously operated, the air impact driver is started and the safety mechanism does not function.